Medicine has not yet figured out how many concussions is too many. The number that leads to permanent deficits in memory, concentration, and other cognitive processes, and/or that increases the risk of dementia and other problems later in life, is likely to be different for each athlete.

Several other factors besides the overall number will help influence a recommendation to an athlete to consider retiring from contact or collision sports, including:

Force. For some athletes, as the number of concussions rises, the force required to produce a concussion seems to decrease. When an athlete reports developing concussion symptoms after a seemingly minor blow to the head (such as an accidental blow to the head by the arm of an opponent or friend, such a sign is concerning, and will prompt a sports concussion specialist to discuss retirement with the athlete.

Slower recovery. Most athletes recover from concussions quickly, in a matter of days to weeks. In high school-aged athletes, nearly 85 percent will be symptom-free within one week of their injury. For some athletes, however, the recovery time is much longer, lasting weeks to months. For others, they recover from their first few concussions quickly, but as they suffer more concussions, the recovery time increases, lasting weeks to months, or in some rare cases, longer than a year.

Pronounced cognitive losses. After a concussion, many athletes lose some cognitive function, e.g. their ability to think, remember, concentrate, and reason, which they regain as they recover, and which is a prerequisite to return to play. For some athletes, the cognitive losses they experience at the time of injury increase with the number of concussions, with their memory, reaction time, and the speed with which they process information (all of which can be measured through pen-and-pencil or computerized neurocognitive testing), becomes much worse.

Complicated family decision

For most athletes, retiring from contact or collision sports has a major impact on their lives.

For elite high school and college athletes trying to make it to the pros, it means giving up their dream.

Even for athletes at the high school level and younger, much social activity, self-identity, and enjoyment comes from sports participation, which, studies show, have many benefits for both boys and girls.

The importance of such participation is often underestimated by clinicians, parents, teachers and other adults.

Often, when an athlete stops playing contact and collision sports, they lose the friends they spend time with before practice, while dressing for sports, stretching, warming up, after practice, while changing and showering, and on the bus ride to games. Not being around during these times means they miss out on conversations, jokes, the latest gossip, and the discussions that make people friends. This can be quite devastating.

The decision to retire should be made jointly, after long discussion between the athlete, the athlete's family, other people important to the athlete (e.g. coach) and the team involved in the athlete's care, including the physician, neuropsychologist, nurse practitioner, and other members of the care team, and takes place over a series of visits lasting weeks to months during which the following is explained to the athlete and others:

There is some risk of long-term problems after multiple concussions;

No one knows how many is too many;

The number is likely different for each athlete;

The athlete can be monitored for the factors noted above, and, if they occur, possible retirement needs to be discussed;

They need to understand that, even without the presence of the concerning factors discussed above, there remains some risk.

They must decide for themselves whether or not they are willing to take that risk.

For those young athletes who do not seek to play professional sports, or who do not have a realistic chance of doing so, most will assume less risk, and will retire from high-risk sports after fewer concussions than prompt an athlete who earns their living by playing professional sports to retire.

Ultimately, athletes make the decisions themselves, and only in very rare cases will a doctor refuse to allow an athlete to return against his or her wishes, and, even then, they are encouraged to seek a second opinion.